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Comment by frankbreetz | Hacker News Reader

Does anyone know the risk of meltdowns for fusion? I know the energy released is several orders of magnitude larger then that of fission(what nuclear power plants do now), so I would assume a meltdown would be several orders of magnitude worse. If we can create energy through fusion safely it really is unlimited carbon neutral energy. There is a limit, but the natural end of the planet will happen first. This is great news! I had no idea we were this close to fusion.

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Fusion reactors cannot melt down, if the containment fails, the plasma will be cold by the time it reaches the walls. Wikipedia has a decent rundown of the various failure modes and resulting effects.

As far as I know there is no risk for a meltdown. The amount of energy/fuel in the reactor at any given moment is pretty low. Unlike with a fission reactor they have to keep feeding the reactor constantly or it will stop.

I could imagine that the fuel supply getting jammed on is not so dissimilar from the control rods being jammed out.

Armchair opinion: I believe the fuel isn't radioactive, and neither is the reaction. So while it could blow up, it's not going to have a ton of radioactive fallout.

Sort of like a fusion bomb, the fusion bomb doesn't produce radioactive fallout, but the fission bomb necessary to start the fusion bomb does.

D-T fusion, which ITER will use, produces neutron radiation, which will cause neutron activation in reactor components, rendering them radioactive: https://en.wikipedia.org/wiki/Neutron_activation

Aneutronic fusion is possible, but requires collision energies much higher than what ITER is designed to create.

In theory you can chose construction materials that don't do anything too horrible when you bang them with neutrons in the construction of your fusion reactor. I'm not sure how well this is achievable in practice in the construction of a fusion reactor but I'm pretty sure they can avoid using anything that creates byproducts as nasty as uranium does.

EDIT: I do wonder if there's a danger of nuclear proliferation with using fusion reactors to breed plutonium from natural uranium, though.

Yes and no. Depending on the fusion process, the fuel can be radioactive: DT for example, Tritium, Hydrogen-3 is a beta emitter. But it certainly isn't as bad of a thing especially compared to Uranium and friends.

EDIT: see other comment, DT produces neutrons which can be bad, I still think the amount nastiness is small compared to say Chernobyl.

> But it certainly isn't as bad of a thing especially compared to Uranium and friends.

Tritium is something like 10 orders of magnitude more radioactive than uranium (in terms of its specific activity). Uranium itself is so low in radioactivity that the bigger risk when handling it is heavy metal poisoning. The by-products of fissions are much more radioactive, though.

The by-products is what I meant when I said Uranium and friends.

If you set of a thermonuclear bomb in the middle of the air you'll get a bit of fallout from the primary stage that sets it off, the secondary, fusion, stage will produce a bunch of neutrons but no fallout, and the third, depleted uranium, stage will absorb all the neutrons from the fusion and produce a fair amount of fallout. A lot of neutrons will escape and interact with the air but none of the elements you find in the atmosphere turns into anything much worse than deuterium or carbon-14 when you whack it with a neutron so overall its fine. You can replace the third stage with lead and lose half the explosive force but gain an almost entirely clean explosion.

If you set it off near the ground many of the escaping neutrons will interact with things that get very angry when activated by a stray neutron. A big bomb detonated near the ground, to crack open a bunker for instance, can send stupendous amounts of fallout into the air and kill people hundreds of miles down wind of acute "vomit now and die in a week" radiation poisoning.

It’s more like the fuel pump of your car breaking.

No. Too many fusion nuclei will destabilize the plasma as quickly as too few.

Fusion is about hitting a precise balance point. It's why it's so hard.

If your supply of Deuterium and Tritium was jammed wide open you, would stop the fusion (either by switching of the heating if you are in startup or by messing with the magnetic confinement to make the plasma hit the diverter). At that point all you do is pumping low density, relatively low radioactivity gases into a large gas proof vacuum vessel. No explosion, no meltdown, no release of radioactivity to the containment building let along the environment. As far as worst case scenarios go it is pretty harmless. The entire ITER site will never store more than 4kg of tritium, most in the form of metal-hydrites.

Too much fuel would estinguish the fire, similar to too much fuel in a car engine will cause it to stall.

The high temperatures necessary for fusion are only attainable at low masses, if too much fuel enters the reactor, it cools down and becomes inert. The fuel itself is not radioactive and safe for consumption in reasonable amounts (heavy water inhibits biological processes if it becomes too much in concentration, but unless you drink straight from the heavy water tanks for days, this is essentially not an issue)

I think the situation is reversed for fusion as it is for fission. Fusions requires fine control and energy input to keep going. Where fission you need fine control and constant energy removal to keep the process from running away.

Also when the plasma in a TOKAMAK escapes and hits the wall it strips off heavy metal ions which radiates all your beam energy away.

Meta, but given frankbreetz seems be just asking a question doesn't mean they have to be downvoted for being ignorant. I don't think they mean to misinform.

Technically the hydrogen bomb is fusion. That's a bit of a meltdown.

It depends on what you mean by worse - as I understand it, a fusion-induced meltdown at least wouldn't release massive amounts of radiation. Massive amounts of heat and destruction to the immediately-surrounding infrastructure? Could be. But not fallout, like caused all the really big problems post-Chernobyl.

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